(See also Overview of Chromosomal Disorders.)
Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body (see Genes and Chromosomes for a discussion about genetics). Genes contain instructions that determine how the body is supposed to function.
An extra chromosome, making three of a kind (instead of the normal two), is called trisomy (see also Overview of Chromosome and Gene Disorders). Children who have trisomy 18 have a third chromosome 18. Trisomy 18 occurs in about 1 out of 6,000 live births. However, many affected fetuses spontaneously miscarry. The extra chromosome almost always comes from the mother. Mothers who are over age 35 are at increased risk of having a child with trisomy 18. There are more girls than boys with trisomy 18.
Symptoms
In the womb, affected fetuses are not typically very active, and there is often excess amniotic fluid and a small placenta.
Physical abnormalities
At birth, newborns are often very small because their muscles and body fat are underdeveloped. Newborns are typically limp and have a weak cry. The mouth and jaw may be small, which gives the newborn's face a pinched appearance. Other visible deformities are common, including a small head, low-set and malformed ears, a narrow pelvis, and a short breastbone (sternum). Physical abnormalities may be obvious at birth. However, some newborns have abnormalities that are not as severe.
The hands are clenched in fists, and the index fingers often overlap the middle and ring fingers. The fingernails are underdeveloped. Skinfolds, especially over the back of the neck, are common. The big toes are shortened and frequently bend upward. Clubfeet and rocker-bottom feet are common.
Internal abnormalities
Internal organs also have defects. Severe abnormalities may be present in the heart, lungs, digestive tract, and kidneys. Boys may have undescended testes. Newborns may also have hernias, muscles that have separated from the abdominal wall, or both.
Diagnosis
Before birth, trisomy 18 may be suspected based on findings detected during an ultrasound of the fetus. Doctors also can do a test to find deoxyribonucleic acid (DNA) from the fetus in the mother's blood and use this DNA to detect an increased risk of trisomy 18. This test is called noninvasive prenatal screening (NIPS) or cell-free fetal DNA testing. If doctors suspect trisomy 18 based on these tests, they often confirm the diagnosis using chorionic villus sampling, amniocentesis, or both (see Testing for chromosome and gene abnormalities).
After birth, the infant's physical appearance may suggest the diagnosis of trisomy 18. To confirm the diagnosis of trisomy 18, the infant's chromosomes are analyzed using a blood test.
Prognosis and Treatment
There is no specific treatment available for trisomy 18. About 50% of children die within the first week, and only 5 to 10% are still alive at 1 year of age. However, recently, children who have trisomy 18 are living longer. These survivors are at increased risk of developing certain cancers, such as a form of liver cancer (hepatoblastoma) and a kidney cancer (Wilms tumor). To help detect these cancers, doctors may recommend that children have periodic blood tests and ultrasonography of the abdomen and kidneys.
Children who survive have severe developmental delay and disability. Family members should seek support.
More Information
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
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Trisomy 18 Foundation: Provides advocacy, education, support, and public awareness programs and services
